1. Field of the Invention
This invention pertains to a method for removing dissolved heavy metals from aqueous liquids containing heavy metals chelated with amino-carboxylic acids. More particularly, the invention pertains to a process for removing dissolved iron and copper from amino-carboxylic acid chelant solutions.
2. Background
In almost any type of metal equipment in which water is evaporated or heat transfer occurs, insoluble salts and corrosion products deposit upon the surface of the equipment to form scale. The composition of the scale depends upon certain factors such as the water quality, the operating temperature, and the type of metal used. For most ferrous-based process units, the deposits typically comprise iron, nickel, lead, and zinc compounds. In addition, copper compounds are commonly deposited with iron oxide in processes that include units fabricated from copper or copper alloy parts, such as condensers and heat exchangers.
Periodically, scale-prone processes are shut down and chemically cleaned to remove the accumulated scale deposits. In most cleaning operations, the scale is contacted with a mineral acid first, to dissolve the hard deposits. Afterwards, an aqueous amino-carboxylic acid solution is typically added to sequester the released metal ions in the form of metal complexes. These complexes are soluble over a wide pH range which facilitates removal of the scale as an aqueous waste effluent.
One problem with this treatment, though, is that once the scale is removed, the resultant chelant/metal liquid is not easily disposed due to the high solubility of the metals in solution. Consequently, there exists a need for a safe and efficient method of removing the metals from amino-carboxylic acid chelate solutions for proper waste disposal.